The invention relates to a method for correcting at least one image defect of a projection objective of a lithography projection exposure machine, the projection objective comprising an optical arrangement composed of a plurality of lenses and at least one mirror.
The invention further relates to such a projection objective.
Projection objectives of the above-named type are used in the lithographic, in particular microlithographic production of semiconductors, in the case of which an object provided with a structure, that is also denoted as reticle, is imaged by means of the projection objective onto a carrier, which is denoted as a wafer. The carrier is provided with a photosensitive layer upon the exposure of which by means of light through the projection objective the structure of the object is transferred onto the photosensitive layer. After development of the photosensitive layer, the desired structure is produced on the carrier, the exposure operation being repeated in multiple fashion in some circumstances.
A projection objective that is used in the case of such a method and has an optical arrangement composed of a number of lenses and at least one mirror is also denoted as catadioptric. Such a catadioptric projection objective is described, for example, in document WO 2004/019128 A2.
Among the high-aperture projection objectives, catadioptric projection objectives are gaining evermore in importance since, by comparison with purely refractive (dioptric) projection objectives, they enable an overall economic compromise for the purpose of fulfilling the manifold customer-specific requirements.
The at least one, and the frequently several mirrors of such catadioptric projection objectives can be subdivided into two classes, specifically those with catoptric power and those without catoptric power. The mirrors with catoptric power serve the purpose chiefly of supplying a dispersion-free catoptric power and a suitable, mostly positive contribution to the correction of the image surface. It is possible thereby to save a number of lenses by comparison with classic, purely refractive designs. Mirrors without catoptric power, which are also termed folding mirrors, serve the purpose of beam guidance and are generally necessitated on the basis of design requirements.
Within the meaning of the present invention, the at least one mirror can be a mirror with or without catoptric power.
A problem with catadioptric projection objectives is the narrow tolerance requirements placed on the optically operative surfaces of the mirrors. These narrow tolerance requirements are caused by the fact that the optical effect of a surface deformation of a mirror is more than twice as large as is the case with the surface of a lens. The reason for this is that a deformation Δz of a mirror is traversed by the incident and the reflected light beams, that is to say twice, while a surface deformation Δz of a lens surface is traversed only once, and moreover a lens has a refractive index of usually >1.
The surface failures of mirrors can be caused by inaccuracies in production, or by layer stresses of the mirror coatings. Failures can also occur in removing and installing mirrors because of the impossibility of always ensuring exact reproducibility of the previous installed position. Deformations owing to layer stresses frequently occur for the fact that the coating and the substrate of the mirror have different coefficients of thermal expansion such that the shape of the mirror is changed upon irradiation with light. A similar effect can occur owing to relaxation processes after coating the substrate of the mirror.
The use of mirrors in projection objectives consequently requires a higher outlay on adjustment in order to do justice to the deformations of the mirrors used that are unknown before the assembly of the projection objective.
Defective mirror surfaces lead to wavefront errors in the projection light, and thus to a defective imaging of the object (reticle) onto the carrier (wafer) that cannot be sustained in view of the currently required miniaturization of the semiconductor structures to be produced.
There is thus a need for a suitable method for correcting such wavefront errors, i.e. image defects of a catadioptric projection objective, and for a catadioptric projection objective that is at least largely error-compensated.
One possibility for compensating image defects of a catadioptric projection objective that are caused by one or more defective mirrors could consist in correcting the defective mirror surface or surfaces directly by means of a local aspherization by polishing or ion beam etching. However, this sometimes turns out to be very complicated, since the defective mirror or mirrors need to be removed and reinstalled from and in the projection objective several times in some circumstances, and this places particularly high requirements on the adjustment of the reinstalled mirror or mirrors. In addition, as already mentioned the optical effect of defects of mirrors is substantially greater than the optical effect of defects of lenses. Moreover, because of engineering in accuracy in the aspherization process on mirror surfaces it is frequently not even possible for the mirror surface to be corrected directly.
An alternative possibility to this end consists in aspherizing an optically operative lens surface in the immediate neighborhood of the relevant defective mirror.
However, this mode of procedure is not possible for all designs of catadioptric projection objectives, since it is not always ensured that lenses are located in the immediate vicinity of the relevant mirror, as is the case with the projection objective in accordance with WO 2002/044786 A2, FIG. 1. Furthermore, the case can also occur where suitable lenses are certainly present but are not suitable for aspherization for reasons of design type.
It follows that there is a further need for a suitable method for correcting at least one aberration of a catadioptric projection objective, and for an error-compensated catadioptric projection objective whose aberrations are caused by defects in one or more mirror surfaces.